The present invention relates to a rust preventive steel sheet excellent in rust prevention, workability, and electrodeposit coatability mainly intended for use in automobile car bodies, electric appliance boxes, furniture, building external walls, and the like.
In recent years, the target quality required for rust prevention of automobiles is becoming increasingly higher year after year. As regards the corrosion restricted service life, the Canadian Code specifies 1.5 years for cosmetic corrosion and 5 years for perforation corrosion. In the U.S.A., development is being forwarded aiming at 5 years for cosmetic corrosion and 10 years for perforation corrosion. To meet such requirements, so-called Zincrometal.RTM. and organic composite plated steel sheets are in use as the surface-treated steel sheet for automobiles.
The zincrometal sheet is based on the technology developed by Diamond Shamrock Co. (U.S.A.) and is prepared by coating a zinc-rich primer Zincromet.RTM. on a steel sheet treated with Dacromet.RTM., an aqueous solution of a mixture of chromic acid anhydride, metallic zinc particles, reducing catalyst, thickener, etc. Although the sheet has good corrosion resistance and electrodeposit coatability, since it is coated considerably thick (10-15 .mu.m) with a primer of high metallic zinc content, it has been pointed out that it is expensive, liable to cause peeling of coating film during forming operation and to give rise to decreased spot welding strength caused by combustion residue formed during welding.
Various organic composite plated steel sheets have been proposed to ameliorate the above-mentioned problems. Thus, zinc-plated or zinc alloy-plated steel sheets are, after subjected (or directly without being subjected) to a chemical conversion treatment such as chromate treatment, coated with a protective film comprising mainly organic substances. In such cases, since increasing the thickness of plating layer leads to such problems as cracking and peeling in forming operations, the measures taken for enhancing the corrosion resistance have been predominantly through the improvement of protective film, and various proposals have been made regarding the composition of the film.
The vehicle resin of protective film commonly used is acrylic resin or epoxy resin. It is known that, generally speaking, acrylic resin is unsatisfactory in solvent resistance, alkali resistance, and adhesion and epoxy resin is unsatisfactory in subsequent workability with respect to its ductility. With the aim of obtaining a better balance of these properties, proposals have been made to use polyurethane type resins as the material for vehicle.
For example, there may be mentioned Japanese Patent Application Kokai (Laid-open) Nos. 56-84,759, 56-89,548, 57-30,717, 58-40,372, 58-52,370, 58-174,582, 60-13,078, 62-73,938, 62-225,341 and 62-289,274. Among them, Japanese Patent Application Kokai Nos. 56-84,759, 56-89,548, 57-30,717, 58-40,372, 58-52,370 and 58-174,582 relate to coating materials prepared from polymers or oligomers containing an active hydrogen-bearing group such as the hydroxyl, carboxyl, and amino group and block isocyanate as starting materials.
Thus, Japanese Patent Application Kokai No. 56-84,759 relates to a "coating composition comprising (a) an oil-free alkyd resin having an acid value of 50 KOH mg/g or less and a hydroxyl value of 50-250 KOH mg/g which has been reacted with 1-15% by weight of a polyphenol-carboxylic acid and (b) a block isocyanate compound.
Japanese Patent Application Kokai No. 56-89,548 [Japanese Patent Publication (Kokoku) No. 62-49,116] relates to "a precoated metal obtained by coating on a metal sheet, followed by heat-curing, a composition comprising bis(isocyanatomethyl)cyclohexane blocked with an oxime-type or lactam-type blocking agent, or an adduct thereof, and a polyol resin". Examples of the substrate metal given therein include phosphate-treated galvanized sheet iron, etc.
Japanese Patent Application Kokai No. 57-30,717 relates to "a process for producing a thermosetting resin which uses a reaction product of an epoxy resin mainly of the bisphenol type, a dicarboxylic acid and a secondary amine having at least one primary hydroxyalkyl group, and a block isocyanate". It exemplifies the use of a coating material comprising said thermosetting resin as the vehicle, as a primer for zinc phosphate-treated galvanized sheet iron.
Japanese Patent Application Kokai No. 58-40,372 relates to "a coating composition for chromate-treated zinc-plated steel sheets comprising an oil-free alkyd resin having an acid value of 50 or less and a hydroxyl value of 50-250 which has been reacted with tannic acid, and a block isocyanate compound."
Japanese Patent Application Kokai No. 58-52,370 relates to "an aqueous coating composition for chromate-treated zinc-plated steel sheets comprising the organic amine salt or ammonium salt of an oil-free alkyd resin having an acid value of 20-100 and a hydroxyl value of 50-250 which has been reacted with tannic acid, and a water dispersible or water-soluble block isocyanate compound."
Japanese Patent Application Kokai No. 58-174,582 relates to a weldable coated steel sheet obtained by applying to a zinc-plated steel sheet, followed by drying, a composition comprising chromic acid anhydride, a water-soluble acrylic resin and/or water-dispersible acrylic resin, an organic reducing agent and water as under coating and then applying thereonto, followed by baking and drying, a top coating composition comprising an oil-free polyester resin containing as the dibasic acid 30-70% by mole of terephthalic acid relative to the total organic acid content, a melamine resin and/or block isocyanate as the curing agent, zinc dust and solvent, in a dry film thickness of 3-20 .mu.m."
Japanese Patent Application Kokai Nos. 56-84,759, 58-40,372 and 58-52,370 employ, as the polyol component to be reacted with isocyanate, an oil-free alkyd resin containing a specified acid value, namely a specified amount of carboxyl groups, which has been reacted with a polyphenol-carboxylic acid or tannic acid. Although film thickness is not particularly specified in these inventions, coated articles of 25.+-.3 m film thickness are used in the comparison of the film properties between the compositions of said inventions and those mentioned as comparative examples, to show the more excellent corrosion resistance given by the compositions of the inventions.
However, in the case of rust preventive steel sheets for cathodic electrodeposit coating intended for in the present invention, thick coating of nonionic or anionic resin gives rise to poor electrodeposit coatability even when the resin shows good adhesion and high hardness and good corrosion resistance is exhibited by thick coating. That is, crater-formed defects are apt to develop in the electrodeposit coated film.
Japanese Patent Application Kokai No. 56-89,548 is characterized by using as block isocyanate a blocked bis(isocyanatemethyl)cyclohexane or its adduct. The isocyanate, however, is more expensive and less reactive than aromatic isocyanates. The polyols used therein are all, except for one example of nonionic acrylic type, anionic polyester polyol. No mention is made of cationic ones.
Japanese Patent Application Kokai No. 58-174,582 uses as the polyol an oil-free polyester of a specific composition. Though it makes no mention of the acid value of polyester, the polyester is usually an anionic polyol having a certain extent of acid value. Further, a coating film thickness of 3-20 .mu.m is specified as a limiting condition. Also in this case, the coating is not suited for use as the primer for cathodic electrodeposit coating for reasons described above. Further, although the use of a block isocyanate as the curing agent is mentioned as the feature of said invention, only those examples wherein methylol melamine is used as the curing agent are given and no mention is made of the use of block isocyanate, in its working examples.
In Japanese Patent Application Kokai No. 57-30,717, the resin used, though its structure is not fully clear from the description of the specification, is cationic and is similar to the vehicle used in the present invention in respect of having a blocked isocyanate group. However, no mention is made therein of cathodic electrodeposition, addition of silica sol and coating film thickness. A film thickness of 8 .mu.m is exemplified in its working example. It is thought that, as described in its specification, the resin has been developed mainly for use in so-called precoated metals of "precoat-top coat" type, which are in use as building materials and the like.